• Daniel Vetter's avatar
    drm: Split connection_mutex out of mode_config.mutex (v3) · 6e9f798d
    Daniel Vetter authored
    After the split-out of crtc locks from the big mode_config.mutex
    there's still two major areas it protects:
    - Various connector probe states, like connector->status, EDID
      properties, probed mode lists and similar information.
    - The links from connector->encoder and encoder->crtc and other
      modeset-relevant connector state (e.g. properties which control the
      panel fitter).
    
    The later is used by modeset operations. But they don't really care
    about the former since it's allowed to e.g. enable a disconnected VGA
    output or with a mode not in the probed list.
    
    Thus far this hasn't been a problem, but for the atomic modeset
    conversion Rob Clark needs to convert all modeset relevant locks into
    w/w locks. This is required because the order of acquisition is
    determined by how userspace supplies the atomic modeset data. This has
    run into troubles in the detect path since the i915 load detect code
    needs _both_ protections offered by the mode_config.mutex: It updates
    probe state and it needs to change the modeset configuration to enable
    the temporary load detect pipe.
    
    The big deal here is that for the probe/detect users of this lock a
    plain mutex fits best, but for atomic modesets we really want a w/w
    mutex. To fix this lets split out a new connection_mutex lock for the
    modeset relevant parts.
    
    For simplicity I've decided to only add one additional lock for all
    connector/encoder links and modeset configuration states. We have
    piles of different modeset objects in addition to those (like bridges
    or panels), so adding per-object locks would be much more effort.
    
    Also, we're guaranteed (at least for now) to do a full modeset if we
    need to acquire this lock. Which means that fine-grained locking is
    fairly irrelevant compared to the amount of time the full modeset will
    take.
    
    I've done a full audit, and there's just a few things that justify
    special focus:
    - Locking in drm_sysfs.c is almost completely absent. We should
      sprinkle mode_config.connection_mutex over this file a bit, but
      since it already lacks mode_config.mutex this patch wont make the
      situation any worse. This is material for a follow-up patch.
    
    - omap has a omap_framebuffer_flush function which walks the
      connector->encoder->crtc links and is called from many contexts.
      Some look like they don't acquire mode_config.mutex, so this is
      already racy. Again fixing this is material for a separate patch.
    
    - The radeon hot_plug function to retrain DP links looks at
      connector->dpms. Currently this happens without any locking, so is
      already racy. I think radeon_hotplug_work_func should gain
      mutex_lock/unlock calls for the mode_config.connection_mutex.
    
    - Same applies to i915's intel_dp_hot_plug. But again, this is already
      racy.
    
    - i915 load_detect code needs to acquire this lock. Which means the
      w/w dance due to Rob's work will be nicely contained to _just_ this
      function.
    
    I've added fixme comments everywhere where it looks suspicious but in
    the sysfs code. After a quick irc discussion with Dave Airlie it
    sounds like the lack of locking in there is due to sysfs cleanup fun
    at module unload.
    
    v1: original (only compile tested)
    
    v2: missing mutex_init(), etc (from Rob Clark)
    
    v3: i915 needs more care in the conversion:
    - Protect the edp pp logic with the connection_mutex.
    - Use connection_mutex in the backlight code due to
      get_pipe_from_connector.
    - Use drm_modeset_lock_all in suspend/resume paths.
    - Update lock checks in the overlay code.
    
    Cc: Alex Deucher <alexdeucher@gmail.com>
    Cc: Rob Clark <robdclark@gmail.com>
    Signed-off-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
    Reviewed-by: default avatarRob Clark <robdclark@gmail.com>
    6e9f798d
i915_debugfs.c 100 KB